It is briefly recalled that a quick-connection coupling includes means for retaining the end of the duct in a bore designed to receive said end. Such means are of two main types: claws, i.e. arms extending substantially parallel to the outside surface of the tube and uniformly distributed thereabout, with the free ends of the arms being provided with respective teeth directed towards the surface of the tube while the body of the coupling includes a camming surface for engaging the opposite surfaces of the arms so as to force the teeth to penetrate into the outside surface of the tube should the tube be subjected to force in the extraction direction; while the second type is constituted by a washer whose inside edge is subdivided into teeth that extend substantially over a conical surface and that bite into the surface of the tube.
In addition to the above retaining means, a quick-connection coupling also includes at least one sealing gasket which is situated beyond the fastening means of the coupling in the direction in which the tube is inserted into the body of the coupling. In most cases, the sealing means is constituted by an O-ring that extends between a cylindrical bearing surface of the bore in the body and the outside surface of the tube that is inserted therein.
An O-ring presents numerous drawbacks. Firstly, when the tube is inserted, the end of the tube tends to come into abutment against the outside surface of the ring and to adhere thereto so as to cause it to rotate about itself, thus subjecting the inside of the ring to local stresses, necking, and deformation that impair the quality of the sealing obtained and also the lifetime of the O-ring. In addition, such an O-ring is poorly adapted to receiving tubes having outside diameters that, for reasons of manufacturing tolerance, vary considerably about a nominal value from one tube to another. Finally, O-rings are highly sensitive to ovalization of the tube received in the coupling, such that ovalization is almost certain to constitute a source of leakage.